Moving target radar display apparatus



Sept 22, 1959 R. MATTHEWS MOVING TARGET RADAR DISPLAY APPARATUS 5 Sheets-Sheet 1 Filed Oct. 12, 1955 Sept. 22, 1959 R. MATTHEWS MOVING TARGET RADAR DISPLAY APPARATUS 3 Sheets-Sheet 2 Filed Oct. 12, 1955 lll/ \ ONGS mmv l/IVF'VTFR R. MATTHEW: @IWW MUN Sept. 22, 19159 R. MATTHEWS 2,905,938

y MOVING TARGET RADAR DISPLAY APPARATUS Filed Oct. 12, 1955 3 Sheets-Sheet Z5 MOVING TARGET RADAR DISPLAY APPARATUS Application October 12, 1955, Serial N0. 540,078

Claims priority, application Great Britain October 13, 1954 11 Claims. (ci. 34a-1.7)

This invention relates to radar display systems of the kind in which echo signals are displayed on a repetitivelyscanned trace. The echoes may kbe displayed as a deflection of the trace as, `for example, in A-scope displays or may be shown as a brightness modulation of a trace Jwhich is moved across a screen as, for example, in a plan-position-indicator display in which a radial trace is rotated in synchronism with an antenna so that the targets are displayed on the screen at distances from and at angular positions about the centre of rotation of the trace corresponding to the actual ranges and bearings.

It is an object of the present invention to enable any selected or all static targets to be cancelled or substantially reduced in amplitude in a radar display.

According to this invention, in -a radar display system having la repet-itively scanned trace there is provided a representation of targets to 'be cancelled from the display or to be reduced in amplitude, scanning means for scanning said representation at the same time as and in synchronism with the display scan to produce corresponding electrical signals and circuit means for applying said signals to the radar display system so as to reduce or cancel the corresponding radar signals. This arrangement enables any selected or all static targets to be reduced in amplitude or cancelled from the screen of the radar display. It will be seen that the invention is equally applicable to displays having a brightness modulated trace and displays having a deflection modulated trace.

In the case of a radar display having a brightness modulated trace on the screen of a cathode ray tube, the scanning means may ybe arranged to produce signals for mixing with the video signals fed to the grid to cathode circuit of the cathode ray tube. These additional signals will have to lbe of opposite polarity to the radar display signals. In the case of a radar display having a deflection modulated trace on a cathode ray tube, the signals from said scanning means may be fed to the appropriate deector means. The primary use of the invention is for the suppression of large areas of signals from static targets such as, for example, from extensive buildings and hills. In such lcases it i-s generally immaterial if the opposing signals do not exactly cancel the corresponding radar display signal-s, particularly in the case of displays with brightness modul-ated traces, since, in that case, the small amplitude or thin edge of the remaining echo serves to indicate the unwanted signal faintly thereby preserving the recognisability of the picture. Provided the radar `receiver and display system is not saturated, the arrangement of the present invention will enable targets which are not on the representation, eg. moving targets, to be seen in the areas otherwise obscured by ground clutter.

For a lbrightness modulated radar display, the afore- ICe mentioned representation is preferably in la form similar' to the radar display. Thus, for example, if the radar display is to be a plan-position-indieator display, then the representation is preferably a similar display showing the targets at distances from and :angular distances about a centre of rotation corresponding to their respective ranges and bearings. It will be appreciated, however, that signals derived from such a representation might be fed to -a B-scope display as well as to a plan-position-indicator display.

The representation, in one arrangement, is in the form of a photograph of the radar display. Such a photograph might be taken when there are no moving targets so as to show on the photograph only static targets. The; photograph may then be scanned by a flying spot scan ner to produce the required electrical signals. The photo graph may conveniently be a negative in the form of a;

slide.

Alternatively, it is possible to use a monoscope hav-- ing a signal plate carrying the required pattern for generat ing video signals corresponding to a predetermined rep-- It will be appreciated, however, that it. would be necessary to have a separate monoscope tube: for each representation that is required and thus the:

resentation.

arrangement would not be so flexible as a system using: photographs scanned by a flying spot scanner.

In another arrangement of the present invention, in'- stead of using a photograph for the representation, there is used, as the representation, the previous scan of a separate radar display having a brightness modulated trace which is moved `across a screen. It would be possible to use the afterglow of a cathode-ray tube display but preferably -a storage tube is used. The writing beam of the storage tube is scanned so as always to be a small distance behind the reading beam, whilst the reading beam is scanned in synchronism with the scanning of the radar display. Thus the reading beam is very nearly one display period (i.e., in the case of a plan position indicator display, one aerial revolution) behind the writing beam. This arrangement will cause the cancellation or reduction from the first display of signals appearing on the previous trace and rwill hence show only moving targets. Such an arrangement permits of indication of targets moving at very slow speeds, for example, down to :about half la mile per hour using a typical radar display system. The lag of the writing beam is preferably just suicient to prevent inter-action of the reading and writing beams. The amount of delay however,

is not critical provided it 4is sufficient to prevent interaction of the -two beams. The required delay in the writing beam may be obtained by feeding the signals through an electrical delay circuit. The trigger pulse for starting the trace for the writing beam is conveniently derived from the 4trigger pulse for the radar display by use of the same delay circuit so as to ensure that the signals displayed by the writing lbeam yappear in their correct positions of the screen no matter what the actual amount of the delay introduced. Provided the delay is small, the position of the trace of the writing beam on the screen of the storage tube (eg. in a plan position indicator display, the angular position and centre of rotation) may be controlled by the same control signals as determine the position of the reading beam trace and the trace of the radar display. Ditferentiating ampliers may be used, if desired, in the video circuits for the radar display and storage tube.

"Ihe following is a description of two embodiments of A the invention, reference being made to the accompanyingdrawings in which Figures l, 2 and 3- are block diagrams of three different forms of radar display apparatus in which static targets are cancelled or reduced in amplitude.

Referring to Figure l there is illustrated diagrarnmatically a pulse radar system having a pulse transmitter of which Ythe p ulse recurrence frequency is controlled byatriggerpulse generatori-l1; The outputv ofi the-- transmitter is radiatedbya scan-ning antenna-M-and the echoes. received fromtargetsare fed from this scanningantenna to a-receiver lidi-Which provides-a video-frequency output. l

The scanningantenna l-Z- is. mechanicallycoupled` toa. magslip. 14 which. resolves. a low ,frequency input signal from an Oscillator l-'into-sine and-cosine components representative ofA the angular position-ofthe antenna.V "Ehe two outputs from the magslip 14 are fed respectively-v totwo demodulators. 15:, 1;-7' which providedirectA voltages proportional tothe sineand cosine-of the-angular position of. the antenna. and thesetwodir-ectvoltages areemployed: respectively to. control`l the. amplitude of! saw tooth-currents generatedhy two sawtooth.time-base gen-v eratQrSdS, lrespectively. The instant ofi starting at the duration of thesawtoothr currents. are. determined by gate pulse from a gate pulse generator 25 which is con-iy trolledzby` the ,trigger pulse-generator 111. A: demodulator ci-rcuit.suitable for usein the .demodulators/l, 17.: is de: scribed inv British patent specilicationNov 679,723- and a..4 saw-tooth generator. for, generating a. saw-tooth currentof,l amplitude proportional to.a direct input-voltage.- is described in British patent speci'lication No. 679-9722; Eachof.y the time-.basefgenerators 18,' 19inthe-arrangef ment of Figure l also-includes a paraphrase-amplitudev` sov that two saw-tooth outputs. orA opposite polarity are..]gxroyided.` rfhetwooutputs fro-mthe .time-.base gen-4 erator ki are-.,fedrespectively to a-pair-of'defleetor-coilsfV 29, Zmfa cathode ray. tube .22, and thetwo outputs: from the time-base generator l9-are fedrrespectively. to. a-second;p.air of: deector-coils 23,- 245 This arrangeL ment thus provides aradial scanon the.- screen-of thecathode ray tube Vwhich scanis.. rotated in synchronisnt with the` rotation of the scanningY antenna, and, ifthe.- video ,output ofthereceiver 13; were applieddireetlyf. to. the` tube lll-to .modulatethe beam thereof, -.a..plan.posi. tion display would be obtained on the screen.

The outputs from-the two dernodulatorsgl, 17 l are-alsofed to a-.sccond pair ofttirne-base` generators 3i?, 3lifwhich are `similar to the units i8, 19, andfwhich Vfeed fourder.

tubef22. The -yingspot scanner isnrrangedo,scannae photograph 37 or otherrepresentation;of thexed'echoes.; which4 it is, required -to cancel or reduce-son; ther screen: o f. 1'

theradar-display. In ,the arrangementA shown ira-,Figure l thephotograph-'i is in the.OYXILQJLHCgaLVQ.and-'the light from the-.ilyingfspot scanner -istransrnittedthere: through, and is-Vpickedy up, by L -photo,-cell StL- y The .outputi from the photo-cell isarnplied .by a .videoffrcquencyani: pliiierV .39 and then appliedytogone-input of.: a, combining.-

unitjlltn.- ToA the other ilpvueof this combiningunituisr fed thevideo signals from the receiver.. 13.3 Thecoinbin-A ing unit is arranged-so. thatV the..signalsirorn-thephoto-i cell 3 8 zare subtracted from the video-.output-of 'the ref ceiver 13 Yto give .a .resultant-output from'the--combining-lunit. iivhichis then. applied between-thegrid and cathode j of. the cathode ray tube, 22 -tomodulate Vthe brightness ,oih

the-.cathode ray tube-beam. it will-lbeseen" that with" this arrangement the flying-spot scanner will: Jrovidey out;- put-signals representative of the Xed echoes as displayed Figure. 3y howevenythe timefbasegenerators 1S., l9zare controlled by means of agate pulse from a gate pulse 4 on the photograph 37 and that these signals are subtracted from the received output so that echoes from static targets-- would be cancelled or substantially reduced in amplitude on the screen of the cathode ray tube 22.

As is shown in Figure 2, which illustrates a modiiication of the arrangement of Figure l, instead of using a flying-spot scanner 36 and photograph 37, a monoscope 45 may be employed having a screen on which a signal plate carrying a required pattern for generating video signals corresponding to a; predetermined representation. inFigure. 2 the same reference numerals areV employed. as in Figure l' to indicate corresponding elements.' TheY onlyy difference, betweenthe arrangementsyof, Figures 1j. and`v` 2:- is;..the.- use in'. Figure ,2r of; the. monoscope 45'. v The time-base generators 3S, 3l, feed the four; deector'- coils 4e, di?, 4S. andi9v which. control, the, deilectionv of the monoscope beam. The output from the monoscope is fed to the amplifier. 39;and-thenee,` after. amplification to the combiner 4S where it'v iscombined with the video signals from the radar receiver i3.. TheA system of Figure 2 operates in a similar manner to that'of Figure l and serves to cancel or reduce in amplitude on the display echoes corresponding; to those represented on-v` the monoscope.

The arrangements oiv Figures lA and- 2-require representations of the-echoes expected from'txedtargets'.v In' practice Vthe echoes from iixedtargets may vary from time-- signals are-derived from thc-previous scan ofthe` radar--v display. Referringfto-Figure there isf-provided, asbeforc, a pulsetransinitter- Elfi-controlled by a trigger pulse generator' il, a scanning antenna l2, a. receiver 13-iand a-magslip M whichis fed by a-low frequency oscillatorl5 and which-,feedsftwo demodulators 116, 17 vtoprovide= direct voltagesproportional tothe sine and-cosine ofthe angular positionof the. antenna. control twotime-base generators 13.', 19-'which in-tum-bymeans ofV coils 2?,- Zl, 23,' dfcontrol.the-.deflection of thebeam of thecathoderay tube 22.- lnl the arrangement of' generatony -wliichdetermines the instant of start? and= duration Ofi: the saw-toothv waveforms. generator proyidcs gate pulses vsynchronised from-the 1received radar,ground-wavesignals, byv means. of aflirni-terz 5lto. which is applied the video output from there-- ceiver 13, the limiter being arranged to separate thc large-.amplitude ground-wave4 from the other Video-sig.. nals.

The demodullators lo, il? also control four further-l time-base generators 52, S3, 54, SS.

generators .52, 53-feed the fourdeflector coils 56, 57,

58,- 59: controlling the reading bearnof a storagertypelcathode ray tube 6@ and the twovtimefbase. generators'. 54, Sfeed deflector-coils 6l, 62, 63, drcontro-lling the,-

writing beainof the. tu-be- 6i?. The time-,base generators 52, 53 are controlled from the gate puls-e generatorf. v but the time-base generators 51%,.. 55A are triggered atH a .short time .afterthe other four timefbase generators, this delay being effected by means of an electrical delayf l'circuit -6'5 into which the video output front-the receiver 13 Jis ,fed. The. video signals, after-passing through the.y delaycircuit, are amplied by an amplifier 66 and fed to a limiter 67 which separates `the large amplitude ground;v wave from thefother video signals. The grotmdwave signals are fed to a gate pulse generator 68 for-generatingV gate pulses-for controlling the start and duration of the saw-tooth waveforms from the time-base gnerators 54,-

55. The output from. the amplifier 66 .fis-` also .used-to modulate the Writing beam of the storage-type cathode lt .will thus be seen that the. time-basesfor-A ray ,tube 60. the writing beam are delayed by4 a-short time-With-:ree spectuto the time-,bases for-fthe. reading beam `andtcjr the display tube Z2 and that the same delay is also applied The two demodulators- This 4 gate. pulse` The two time-basel.

to the video signals which are used to modulate the writing beam. The output from the reading beam of the tube 60 is amplified by an ampjier 7l) and fed to a combiner 71. This combiner 7l is a subtraction unit in which the output from the amplier 70 is mixed with the video output from the receiver 13 so that the signals from the amplifier 70 are subtracted from the received signals from the receiver 13 to give an output representing the difference between these two sets of signals. The output from the combiner 71 is applied between the grid and cathode of the tube Z2 to modulate the brightness of the beam thereof.

The storage tube 60 is preferably of the type in which the reading beam removes any stored signals but alternatively it is possible to provide a separate cancelling circuit for cancelling all stored signals one per revolution of the antenna. Y Y' Y Y Heretofore in systems using a storage tube for delaying information by a complete cycle of antenna rotation it has been necessary to write the infomation on the storage tube in one cycle of antenna rotation and then to read off during the next cycle. While infomation is being read off, it is not possible to record further incoming information on the storage tube. Such an arrangement thus enables only half the incoming information to be employed, the information being recorded in one cycle and read off in the next. While it is being read off, no use can be made of the incoming radar information. This is a very serious disadvantage since it means losing half the possible information from the radar apparatus. To overcome this difliculty it has been proposed in the past in such systems to use two storage tubes and switching means so that information can be written on to one storage tube while it is being read ofi the other, the two storage tubes being read in alternate cycles. The arrangement of the present invention avoids any necessity for two storage tubes and enables all the incoming information to be utilized while employing only a single storage tube.

Considered from another aspect, the storage tube of the present invention provides a delay system into which information can be continuously fed and from which the delayed information may be continuously extracted, the delay system giving a delay of exactly one cycle of antenna scanning. This delay is obtained very accurately without however any necessity for any precision stabilized delay circuits such as are commonly used in many types of moving target display radar apparatus.

It will be seen that, in the arrangement of Figure 3, the writing beam of the storage tube is scanned so as to be always a small distance behind the reading beam, whilst the reading beam is scanned in synchronism with the display tube 22. The required information to be stored in the tube is applied to the writing beam with a small delay introduced by the delay unit 65. The delay period, which is a fraction of a cycle of the antenna rotation, is not critical and is merely such as to ensure that there is no interference between the reading and writing beams. Thus, provided adjacent traces on the storage tube do not overlap, the reading beam is very nearly one display period (that is to say one antenna revolution in the case of 1a plan-position display) behind the writing beam. The beams are scanned in synchronism except that the synchronizing signals for the waveform generators 54, 5S for the writing beam are delayed by the same delay as is applied to the information to be written on the storage tube 60. The reading beam thus moves across the screen slightly ahead of the writing beam and so it requires nearly a complete cycle of the antenna rotation (assuming a rotating antenna radar system) for the reading beam to read information off the storage tube after it has been put on the writing beam. In other words the writing beam is nearly one complete display period behind the reading beam, the difference from one complete display period being the scanning delay deliberately introduced as described above by the delay unit 65. This scanning delay is the same as the delay introduced in the radar information applied to the writing beam and hence the information read from the storage tube 60 by the reading beam is exactly a complete cycle of antenna rotation behind the incoming radar information. This information read by the reading beam is combined with the directly received radar signals in the combiner '7l so as to reduce in amplitude or cancel from the input to the tube 22 signals appearing in the same position on two successive cycles of antenna rotation. However, even if adjacent traces on the storage tube should overlap and the delay in the writing beam is such that the reading beam reads the immediately previously written trace, then the arrangement will operate to reduce in amplitude or cancel signals appearing on the previous ntrace. fin either case-the display will show moving targets whilst reducing in amplitude or cancelling static targets. It will be particularly noted that this arrangement only requires a short delay, the period of which is not critical and this delay (which might, for example, be of the order of 10-30 microseconds) may readily be produced by the electrical delay circuit 65. The video signals and the gate pulse for the writing beam time-bases are delayed by exactly the same amount since the gate pulse is obtained from the video signals, and it will be noted that the gate pulses for the time-base generators 18, i9, 52 and S3 are derived from the received signals in the same manner as the gate pulses for time-base generators 54, 55 so ensuring that there is no difference in the timing of the gate pulses apart from the delay introduced by the delay circuit 65.

In the arrangements of each of Figures l to 3, the representation is made similar to the required display. This is not essential but it is preferable since it enables similar scanning systems to be employed thus facilitating the obtaining of exactly similar scanning. Furthermore all the time-base generators in each of three embodiments are controlled in amplitude of saw-tooth waveforms by direct voltages obtained from demodulating resolved cornponents obtained from a magslip; this arrangement further assists in enabling a high degree of similarity between the various scans to be obtained.

I claim:

1. A radar display system for `displaying radar information from a radar set having a rotating antenna which display system comprises a display cathode ray tube, a storage tube having a screen on which signals are stored by a writing beam and from which signals are read by a reading beam, scanning means for Scanning the reading beam in synchronism with the incoming radar information, scanning means for scanning the writing beam in synchronism with the incoming radar information but delayed by a period corresponding to a fraction of a cycle of the antenna rotation with respect to the scanning of the reading beam to prevent interaction of the two beams, an input circuit for applying part of the incoming radar information to modulate the writing beam of the storage tube, a delay unit in said input circuit for delaying the incoming radar signals for said fraction of a cycle, circuit means for applying the signals read by said reading beam so as to reduce the corresponding undelayed part in the next cycle of the incoming radar information on the display cathode ray tube and scanning means for controlling the scanning of the beam of said display cathode ray tube in synchronism with the incoming radar information and exactly in phase with the scanning of the reading beam.

2. A radar display system as claimed in claim l wherein said circuit means comprise combining means for combining in a subtractive manner an undelayed part of the incoming radar information with signals read by said reading beam and means for applying the combined signals to said display cathode ray tube to modulate the beam thereof.

3. A radar display system as claimed in claim 2 and having a variable gain amplier for amplifying ythe signals readby the reading beam whereby the relative amplitudes ofthe signals fed to said combining means may beradjusted to cancel static targets on said display cathodev ray tube.

4. A radar display system as claimed in claim 1 wherein common scanning control circuits are provided for controlling the scanning ratesV of the reading beam, the writing beam and the beam of the display tube.

5. A radar display system for displaying radar information from a pulse radar set having a rotating antenna which display system comprises a display cathode ray tube, a storage tube having a screen on which signals are stored by a writing beam and from which signals are .read by a reading beam, scanning means for scanning the reading beam in synchronism with the incoming radar information, a delay unit for delaying the incoming radar signals for a period corresponding to a fraction of a cycle of the antenna rotation, an input circuit for applying the delayed radar information from said delay unit to modulate the writing beam of the storage tube, scanning means controlled by the received ground wave signals after passing through said delay unit for scanning the writing beam in synchronism with the incoming radar information but delayed by said fraction of a cycle with respect to the reading beam, which fraction of a cycle is suiicient to `prevent interac-tion of the reading and writing beams, circuit means for applying the signals read by said reading beam and the corresponding undelayed incoming radar information on the display of said display cathode ray tube so that corresponding signals in successive cycles cancel one another and scanning means for controlling the scanning of the beam of said display cathode ray tube lin synchronism'with the incoming radar information and exactlyinphase therewith.

`'6. A radar display system Vfor displaying radar info-rmation from a pulse radar set lhaving a rotating .antenna which .display fsystern. comprises Ha display cathode ray tube, a storage tube having a screen on which signals-are stored by a writing beam and from ywhich signals areread by a reading beam, scanning means controlled by the incoming received ground wave signals for scanning the reading beam in synchronism with the incoming vradar information, a delay unit for delaying the incoming radar signals for a period corresponding to a fraction of'la cycle of the antenna rotation, an input circuit for applying the delayed radarY information from said delay unit to -mod-V ulate the writing beam of the storage tube, scanning means controlled by the received ground wave signals after passing through said delay unit for scanning'the writing 'beam in synchronism with the incoming radar 1i11- formation but delayed by said fraction of a cyclewith respect to the reading beam, which fraction -of a -cycle Ais sufficient to prevent interaction of the reading and writing beams, circuit means for applying the signals read byv said reading beam and the corresponding undelayed incoming radar information on the display of said display cathode ray tube so that corresponding signals in successivecycles cancel one another and scanning means controlled by the incoming undelayed received ground wave signalsv position display, radar information from a pulse radarV set having a rotating antenna which displayl system ycomprises a ldisplay cathode ray tube, a storage tube having a lscreen on Vwhich signals are stored by awriting Vbeam and from which signa-ls areread by a reading beam, resolving means coupled to said antenna for producing sine. and ycosine ydirect voltages, representative. of the angular position of the antenna, a :first pair of s'awtooth Waveformigenerators controlled :by said sine and cosine voltages `from said resolving means to produce respectively two sawtooth waveforms Aof tamplitu'desproportionalto -saidsine and cosine voltages and synchronised to;y start fwith fthe: ztransmitted. radar-v pulses, vfirst scanning means :coupled to said rst pairof sawtooth waveform generators Vfor scanning the reading beamfin synchronism with the incoming radarinformation, a delaynunitl for delaying :the incoming radar signals :for a Vperiod corresponding to a fraction lof a cycleof the `antenna roftation, an input circuit for applying .the delayedradar .information to modulate .the `Writing beam of the storage tube, a second pair .of sawtooth waveform .generators controlled by said sine and cosine voltageslfromwsaid resolving vmeans to produce respectively `two sawtoo'th waveforms of amplitudes proportional to saidgssine-.and cosine voltagesand. synchronised by the'received :ground wavef'signals after passing through said ydelay Vunitgto start afterzthe'conrespondingoutputs of saidlirstpair of salwtooth waveform -generators with the delay ,equal to the; :aforesaid fractionfof `a cycle, Iwhich fraction f a ycycle sis suicient topreventinteraction'of the reading, and writing beams, second scanning means coupledto-said secondipair; of vsawtcoth-waveform generators Ifor scanning theflwriting beam in synchronism with thezdelayed radar information,.circuit. `means for substractively combining'fthe'signals read by said reading beamand .ther corresponding undelayed incoming radar yinformation signals inthewnextcycleof aerial rotationand for applyingzthe combined .signals toA modulate vthe beam of said display cathode :ray tube, `a third .pair `of sawtooth waveform generators'controlled by said sine Aand cosine voltages from said resolving means v. to produce respectively'two sawtoothwave. forms of amplitudes proportional to vsaid sinei'and cosine voltages and synchronised .to start-with the transmittedradar Vpulses and third scanning imeans coupled to saidthird pair of sawtooth waveform-.generators for scanning the beam of said display cathode'ray tube in synchronism with the incomingzradar information and exactly in phase therewith` '9. VA radar display system asV claimedinclainr vwhere insynchronising means are provided'lfor synchronising the `first and third pairs of waveform generators with the incoming undelayed received ground wave signals.

l0. A radar. display system ascIaimed'in claim 8 wherein .said circuit means for subtractivelyccombining the signals read by said reading beam and the corresponding undelayed incoming radar information signals includesan adjustable gain amplifier for amplifying `one of the signals to be combinedv whereby the relative pro portions ofthe signals when combined maybe adjusted.

ll. A radar display Vsystem for displaying radar information from a radar set having a repetitively scanning antenna system,.which display system comprises -a display `cathode ray tube, a: storage tube having a. screen on which signals are stored by a writingbeam and from which'signalsare readby a reading beam, scanning means for scanning the reading beam in synchronism with the incoming radar information, scanning meansfor scanning -the writing beam in synchronism with the incoming radar information but delayed with respect Ato the scanningv of the reading beam by a period correspondingto a fraction of a cycle of .the antenna scanning toaprevent interaction of the reading and `writing beams, aninput circuit .for applying-.part of .the incoming radarinformationto-,modulate the writing beam of the ystorage tube, adelay unit in said inputfcircuit for delaying=theincom ingsi'gnals forY said. fraction of a cycle, circuit rmeans for applying the signals read by said reading beam' s0 as to lreduce the corresponding undelayedV part ofthe next cycle of the incoming radar information on the display ofvsaid display cathode ray tube and scanning means for controlling the .scanning o-f the beam of said display cathode ray Vtube in synchronism withvthe incom- 9 10 ug radar information and exactly in phase with the scan- FOREIGN PATENTS 11mg 0f hereadmg beam- 644,736 Great Britain ocr. 1s, 195o References Citd in the le 0f this patCnt UNITED STATES PATENTS 5 Jensen: abstract of application Serial Number 161,661, 2,667,634 Hart Ian. 26, 1954 published Nov. 20, 1951, 652 O.G. 892, 893. 2,667,635 Hergenrother Jan. 26, 1954 r 2,668,286 White Feb. 2, 1954 2,716,203 Sen et al Aug. 23, 1955 

